1. Field of the Invention
This invention relates to apparatus which responds to electrical signals having a wide dynamic range, particularly the currents and voltages in an electric power distribution system. It also relates to such apparatus which responds to voltages of wide dynamic range in three-phase electrical systems without a neutral.
2. Background Information
Electrical apparatus such as monitors, meters and analyzers used with electrical power systems often must respond to signals with wide dynamic ranges. This is also true of circuit interrupters used in such systems which also incorporate metering functions as well as protection functions. Such electrical apparatus commonly employs ranging circuits to handle the wide dynamic ranges of the input currents and voltages. These ranging circuits typically utilize multiplexers to switch signals from a resistor ladder into the input of an operational amplifier. However, with the ranging in the input circuit of the operational amplifier, the maximum input voltage must be designed to be less than the voltage rating of the analog switches of the multiplexer. This means that small input voltages must be measured at the low end for signals with wide dynamic range, and over-voltage protection for the analog switches must be added. Attempts to place the multiplexer in the feedback circuit of the operational amplifier have produced inaccurate results because the analog switch resistances became pan of the gain determining resistance.
There is a need therefore, for improved apparatus which responds to electrical signals of wide dynamic range.
More particularly, there is a need for such improved apparatus which does not require that the switches of the ranging circuit be able to withstand the maximum voltage produced by the wide dynamic range input signal.
There is a further need for such improved apparatus which can include the multiplexer switches in the output circuit of the operational amplifier without generating inaccuracies in the conditioned signal.
There is a further need for such improved electrical apparatus responsive to wide range signals in three-phase electric power systems.
There is also a need for such improved apparatus which can provide accurate ranging for phase-to-neutral voltages in three-phase electrical systems without a neutral.